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Divergence and Curl of a Vector Field

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Divergence and Curl of a Vector Field Multivariable Calculus Math 212 Spring 2015 Fowler 309 MWF 9:35am - 10:30am CC BY: $n Ron Buckmire http://faculty.oxy.edu/ron/math/212/15/ Worksheet 27 TITLE Divergence and Curl of a Vector Field CURRENT READING McCallum, Section 19.3 and 20.1) HW #12 (DUE TUESDAY 04/28/15 5PM) McCallum, Section 18.4: 1, 2, 3, 4, 15, 16, 20, 23*. McCallum, Chapter 18 Review: 1, 2, 8, 15, 16, 17, 26, 45. McCallum, Section 19.3: 1, 2, 3, 4, 6, 11, 27, 28. McCallum, Section 20.1: 3, 4, 7, 13, 14, 28. SUMMARY This worksheet discusses the geometric and algebraic definitions of the curl and divergence of a vector field. RECALL @F @F Given a vector field F~ in 2 such that F~ = F (x; y)^i + F (x; y)^j the expression 2 − 1 is R 1 2 @x @y called the scalar curl. DEFINITION: scalar curl in R3 ~ ^ Given a 3-D vector field with only two components F (x; y) = F1(x; y)^i + F2(x; y)^j + 0k we can define the (badly-misnamed) scalar curl of F~ to be @F @F curl F~ = 2 − 1 k^ @x @y NOTE: The curl of a 2-D vector field will either be pointed into the page (using the symbol ⊗) or out of the page (using the symbol ) or be the zero vector ~0. The Curl Of A Vector Field DEFINITION: vector curl in R3 The curl of a vector field F~ (~x) in R3 is a vector property denoted by curl F~ (~x) and defined as @ @ @ r~ × F~ where F~ = F ^i + F ^j + F k^ in 3 and r~ is the vector operator ^i + ^j + k^. 1 2 3 R @x @y @z @F @F @F @F @F @F curl F~ = r~ × F~ = 3 − 2 ^i + 1 − 3 ^j + 2 − 1 k^ @y @z @z @x @x @y Algebraically, you could think of the curl as the following determinant: ^i ^j k^ @ @ @ curl F~ = @x @y @z F1 F2 F3 NOTE: The curl is the cross product of the gradient operator with a vector field F~ , so it is a vector quantity. 1 Multivariable Calculus Worksheet 27 Math 212 Spring 2015 The Curl Of Some Of Our Favorite Vector Fields EXAMPLE Let’s find the curl of some of our favorite planar vector fields. A~(x; y) = (x + y)^i + (x − y)^j B~ (x; y) = x^i + y^j C~ (x; y) = x^j D~ (x; y) = −y^i E~ (x; y) = y^i − x^j F~ (x; y) = −y^i + x^j 2 Multivariable Calculus Worksheet 27 Math 212 Spring 2015 Geometric Understanding Of Curl DEFINITION: circulation density The circulation density of a smooth vector field F~ around the direction of a unit vector n^ is defined, provided the limit exists, to be I ~ F · d~x ~ ~ C Circulation of F around C ~ ~ circn^F = lim = lim = (r × F ) · n^ Area!0 Area inside C Area!0 Area inside C where C is a closed curve in a plane perpendicular to n^ positively oriented using the right-hand rule. (When the Right-Hand Thumb points in direction of n^ Other Fingers are curled in direction of traversal around C.) CONCEPTUAL UNDERSTANDING OF CURL The direction the vector curl of a vector field F~ points in is the direction for which the circulation density of F~ is the GREATEST. The magnitude of the vector curl of a vector field F~ is the circulation density of F~ around the direction r~ × F~ points in. If the circulation density is zero around every direction then we say the curl is ~0 and describe such a vector field as irrotational. Recall that gradient fields have the property that every line integral around a closed path is zero so this means that all gradient fields are irrotational, which can be expressed mathematically as r~ × r~ φ = ~0 for any potential function φ Divergence of a Vector Field DEFINITION: divergence The divergence of a vector field F~ (~x) is a scalar property denoted by divF~ (~x) defined as the trace of the Jacobian matrix, i.e. the sum of the diagonal elements of this matrix. In particular, if one @ @ @ considers F~ = F ^i + F ^j + F k^ in 3 where r~ = ; ; then the divergence of F~ can 1 2 3 R @x @y @z be defined as @F @F @F divF~ = r~ · F~ = 1 + 2 + 3 @x @y @z NOTE: The divergence is the dot product of the gradient operator with a vector field F~ , so it is a scalar quantity. GROUPWORK Find the Divergence of the six vector fields depicted earlier on this worksheet. 3 Multivariable Calculus Worksheet 27 Math 212 Spring 2015 Properties Of Gradient, Divergence and Curl As Vector Calculus Operations ~ ~ The divergence r · and curl r × can be thought of as differential operations that are applied ~ to vector fields, and produce scalars and vectors, respectively. The gradient operator r is applied to a scalar function and outputs a vector field. Given scalar functions φ and and vector fields F~ and G~ the following properties apply to the gradient, curl and divergence operators. Distributivity r~ · (F~ + G~ ) = r~ · F~ + r~ · G~ r~ × (F~ + G~ ) = r~ × F~ + r~ × G~ r~ (φ + ) = r~ φ + r~ Product Rules r~ · (φF~ ) = F~ · (r~ φ) + φr~ · F~ r~ × (φF~ ) = φ(r~ × F~ ) + (r~ φ) × F~ r~ (φψ) = (r~ φ) + φ(r~ ) Repeated Applications (“Second Derivatives”) r~ · (r~ × F~ ) = div curl F~ = 0 r~ × (r~ φ) = curl grad φ = ~0 r~ · (r~ φ) = r2φ = ∆φ = div grad φ r~ × (r~ × F~ ) = r~ (r~ · F~ ) − r2F~ = curl curl F~ Exercise How many possible binary arrangements of div, grad and curl are there? QUESTION: How many of these are well-defined operations? How many of these are identically zero? EXAMPLE @ @ @ For F~ = F ^i + F ^j + F k^ and r~ = ^i + ^j + k^ let’s confirm the vector calculus identities 1 2 3 @x @y @z div curl F~ = r~ · (r~ × F~ ) = 0 and r~ × (r~ φ) = curl grad φ = ~0: 4.
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